Saturday, January 21, 2012

What Causes Thyroid Cancer ?

Although scientists have found
that thyroid cancer is linked with a number of other conditions (described in
"What are the risk factors for thyroid cancer?"), the exact
cause of most thyroid cancers is not yet known. Researchers have made great
progress in understanding how certain changes in a person's DNA can cause
thyroid cells to become cancerous.

DNA is the chemical in each of our cells
that makes up our genes – the instructions for how our cells
function. We usually look like our parents because they are the source of our
DNA. However, DNA affects more than how we look. It also can influence our risk
for developing certain diseases, including some kinds of cancer.

Some genescontain
instructions for controlling when our cells grow and divide. Certain genes that
speed up cell division or cause cells to live longer than they should are
called oncogenes. Others that slow down cell division or cause
cells to die at the appropriate time are called tumor suppressor genes.
Cancers can be caused by DNA changes that turn on oncogenes or turn off tumor
suppressor genes.

People inherit 2 copies of each
gene – one from each parent. People can inherit damaged DNA from one or both
parents, which accounts for inherited cancers. Most cancers, though, are not
inherited. In these cases, a person's DNA is damaged by exposure to something
in the environment, like smoking or radiation. Other DNA changes may just be
random events that sometimes happen inside a cell, without having an external
cause.

Papillary Thyroid Cancer: Several DNA mutations have been found in
some forms of papillary thyroid cancer. Many of these cancers have changes in
specific parts of the RET gene. The altered form of this gene,
known as the PTC oncogene, is found in about 10% to 30% of
papillary thyroid cancers overall, and in a larger percentage of these cancers
found in children and/or linked with radiation exposure. These RET mutations
usually are acquired during a person's lifetime rather than being inherited.
They are present only in cancer cells and are not passed on to the patient's
children.

Many (30% to 70%) papillary
thyroid cancers contain a mutation of the BRAF gene. The BRAF mutation
is less common in thyroid cancers in children and in those thought to arise
from exposure to radiation. Cancers with BRAF changes tend to
have more aggressive growth and a greater likelihood of spreading to other
parts of the body.

Both BRAF and RET/PTC changes
are thought to cause cells to grow and divide. It is extremely rare for
papillary cancers to have changes in both the BRAF and RET/PTC genes.
Some doctors now advise testing papillary cancer samples for these gene
mutations, as some studies have suggested they may affect a person's prognosis
(outlook).

Changes to other genes have also
been tied to papillary thyroid cancer, including those in the NTRK1 gene
and the METgene.

Follicular Thyroid Cancer: Acquired changes in the RAS oncogene
have a role in causing some follicular thyroid cancers.

Anaplastic Thyroid Cancer: These cancers tend to have some of the
mutations described above and often have changes in the p53 tumor
suppressor gene and the CTNNB1 oncogene as well.

Medullary Thyroid Cancer: People who have medullary thyroid
carcinoma (MTC) have mutations in different parts of the RET gene
compared with papillary carcinoma patients. Nearly all patients with the
inherited form of MTC and about 1 of every 10 with the sporadic (non-inherited)
form of MTC have a mutation in the RET gene.

Most patients with sporadic MTC
have acquired mutations present only in their cancer cells. Those with familial
MTC and MEN 2 inherit the RET mutation from a parent. These
mutations are present in every cell of the patient's body and can be detected
by testing the DNA of blood cells.

In people with inherited
mutations of RET, one RET gene is usually normal
and one is mutated. Because every person has 2RET genes but passes
only one of them to a child (the child's other RET gene comes
from the other parent), the odds that a person with familial MTC will pass a
mutated gene on to a child are 1 in 2 (or 50%).